The present invention relates to a high-stability piezoelectric oscillator. In particular, the present invention relates to a high-stability piezoelectric oscillator in which, by using a peltier element as the temperature regulating element of the inner oven of a double oven, its temperature can be set to be lower than the outer oven.
Because high-stability crystal oscillators (OCXO) have an excellent frequency precision, frequency temperature characteristic, frequency aging characteristic and the like, it has various uses, from mobile wireless base stations to high precision measuring devices. In order to prevent changes in oscillation frequency even with fluctuations in the ambient temperature, the high stability crystal oscillator has a construction in which the crystal oscillator and the oscillator circuit are housed inside an oven. For a high stability crystal oscillator requiring even greater precision, a double rotation cut SC cut crystal oscillator or IT cut crystal oscillator are used. This results in a crystal oscillator with excellent stress sensitivity and heat impact resistance as compared to when an AT cut crystal oscillator is used.
Referring to FIG. 2, with the horizontal axis as frequency and the vertical axis as reactance, this figure shows the reactance characteristic near the resonance of the SC cut crystal oscillator. The main oscillation, which has the lowest frequency of the three modes that are excited, is the thickness slide oscillation mode (C mode). At higher frequencies than this vibration mode, there are a thickness torsion mode (B mode) and a thickness height mode (C mode) and the like. The resonance frequency (f2) of the B mode, which is adjacent to the main vibration C mode (resonance frequency f1), is present at an approximately 9-10% higher frequency than the resonance frequency f1 of the C mode, and as a result, there are various strategies in the oscillation circuit to avoid the frequency jump phenomenon.
Referring to FIG. 3, this shows the frequency temperature characteristic of the SC cut oscillator (C mode). The horizontal axis is temperature (degrees C.), and the vertical axis is the standardized frequency change (Δf/f). There is presented a third order curve with an inflection point temperature (Ti) of approximately 95 degrees C. The peak temperature Tp, which is at a lower temperature than the inflection point temperature and indicates a zero degree coefficient, is dependent on the double rotation angle that is cut, and the set range for the peak temperature Tp is approximately 65-81 degrees C. Therefore, by setting the temperature inside the oven to be near the peak temperature Tp, a stable frequency is achieved.
For example, if the usage temperature range for the high-stability crystal oscillator is in the range of 0 to 50 degrees C., the oscillator is constructed so that the temperature inside the oven is maintained at a temperature 10 to 20 degrees higher than the upper limit of the usage temperature of 50 degrees C., for example 70 degrees C. By using a SC cut crystal oscillator with a peak temperature Tp of around 70 degrees C., a stable frequency that is not affected by the temperature around the oscillator is outputted.
Referring to FIG. 4, there is a cross-section showing a construction of a highly stable crystal oscillator. An SC cut crystal oscillator 22, an electric part 23 for oscillation, a temperature sensor 24, and the like are mounted onto a printed board 21, and this is housed inside an oven 25. A heater 26 is wrapped around oven 25. From printed board 21, a power source terminal 27 and an output terminal 27 and the like pass through oven 25 and a base 28 in an insulated manner using a hermetic terminal and the like. Furthermore, base 28 is covered by a case 29, and the joining section is sealed by soldering or the like. The operation of the oven is to have current flow in heater 26 by applying voltage to terminal 27 and to maintain a constant temperature for oven 25 by a temperature sensor and a control circuit.
In recent years, there has been a demand for high-stability crystal oscillators that have stable operation even when the ambient temperature is high. For example, when the usage temperature range is from 0 degrees C. to 85 degrees C., it is necessary to set the temperature of the oven to approximately 95 degrees C. which is a temperature higher than the upper limit of the usage range of 85 degrees C.
However, the standard for electric parts of oscillating circuits is mostly around 85 degrees C., and use at higher temperatures is not guaranteed. Although there exist parts for military and satellite uses that can be used at high temperatures, these are very expensive. Furthermore, crystal oscillators have accelerated aging at high temperatures, and there is a risk of having frequency shifts and the like. In addition, the peak temperature Tp of SC cut crystal oscillators is approximately 81 degrees as the upper limit. It cannot be set to be any higher.
In Japanese Laid Open Patent Number 3-104404, there is proposed a high stability crystal oscillator using a peltier element (an element that uses the peltier effect, in other words, the phenomenon, when current flows through a junction point of different types of conductor (or semiconductor), by which there is generation of heat or absorption of heat. The amount of heat generation or heat absorption is proportionate to the current, and when the direction of current is reversed, the generation and absorption of heat is reversed). For example, if the usage temperature range is from 0 to 85 degrees C., when the temperature inside the oven is set to 70 degrees C., the oven is heated or cooled by the peltier element and temperature control circuit in order to maintain 70 degrees C. With this, an SC cut crystal oscillator of a Tp=70 degrees C. is used as the crystal oscillator.
Referring to FIG. 5, there is a cross-section showing the construction of a high stability crystal oscillator using a peltier element. A SC cut crystal oscillator 32, an electric part 33 for oscillation, a temperature sensor 34 and the like are mounted on a printed board 31, and this is housed in an oven 35. A peltier element 36 is attached to oven 35. From printed board 31, a power source terminal 37 and an output terminal 37 and the like pass through oven 35 and a base 38 in an insulated manner using a hermetic terminal or the like. Furthermore, base 38 is covered by a case 39, and its joining section is sealed by soldering or the like. By running current through peltier element 36 through power source terminal 37, the temperature of oven 35 can be maintained at a specified temperature, for example at a constant 70 degrees C.
However, with the high-stability crystal oscillator using a peltier element as shown in FIG. 5, when the ambient temperature is approximately 70 degrees C., it is extremely difficult to maintain an oven temperature of 70 degrees C. because of heat capacity and the like. It is well known that temperature control of the oven is more stable and easier to control when the temperature is set at a high temperature of at least 10 to 20 degrees C. different from the ambient temperature.